Nondestructive inspection (NDI) of a structure involves thoroughly examining the structure without harming it or significantly disassembling it. Nondestructive inspection is commonly used in the aircraft industry to validate the health (e.g., integrity and fitness) of aircraft structures.
NDI may be performed on stiffened composite parts of an aircraft. A stiffened part may have flat areas and corners. A corner is referred to as a “corner radius.” The stiffened part may be made of a composite material such as carbon fiber reinforced plastic (CFRP). A fuselage stiffener is but one example of a stiffened part.
Structural health of a stiffened part can be determined non-destructively by ultrasonic testing. A corner radius of a stiffened part can be inspected ultrasonically by a probe including a radiused shoe that holds an ultrasonic transducer. During NDI, the shoe's radius is pressed against a corner radius of the part, the transducer is acoustically coupled to the part (e.g., with water), and the shoe is slid along the corner radius. As the shoe is being slid, the transducer operates in pulse/echo mode to generate sound pulses, which are transmitted through the corner radius. Reflected sound pulses indicate whether the corner radius contains a crack, void, delamination, etc.
A problem can arise during NDI of a stiffened part if a corner radius of the part is not formed by concentric inner and outer radii (see, for example inner and outer radii R1 and R2 in FIG. 1). The part might be structurally sound, but a corner radius having non-concentric radii might produce a false negative during NDI. For instance, the inspection might falsely indicate a structural inconsistency such as porosity or delamination.
If a structural inconsistency is indicated, additional testing is performed. If the additional testing reveals a healthy structure, then the time and cost of performing the additional testing was wasted.
False negatives can also significantly slow the flow of production. The time and cost of additional inspection requires the production facility to be “occupied” by the part while undergoing additional testing. This can pose a problem if other parts have to wait for inspection.
Consider aircraft stiffeners. Given the number of features to test on each stiffener, the length of each stiffener, the number of stiffeners in an aircraft, and the number of aircraft being manufactured at any given time, there might be miles of stiffener to inspect. The time and money wasted on false alarms, and the slowdown in production, can be significant.
It would be desirable to reduce the occurrence of false negatives.